Which makes the game quite tedious and repetitious. Not only is a "safe" location NOT safe, it is almost always repopulated with almost exactly was there before. (Excluding, perhaps, the initial Boss opponent.) If the site was populated by Supermutants, the replacement population will be Supermutants. If it had been ghouls, it will be ghouls. If it was Raiders, it will be Raiders again. Not only will the replacements be the same, they tend to be in the same location within the site as the originals. This makes it sooooo easy to sneak up on the replacement population: "I need to be careful about going around the corner here because there is a Raider in Power Armor stationed there. Better to snipe that guys from over there where there's good cover and a decent line of sight."
Logically -- if the term is even usable in a video game -- why would the replacements choose to move in to the site of ______'s Last Stand? I can just see a group of Raiders coming upon a location strewn with Raider bodies and thinking to themselves, "Hmm. Something or someone just wiped out the Raiders that were here before us. This is an ideal site for us to set up camp! We won't even have to change any of the defenses!"